Ed Yong in Not Exactly Rocket Science:
Across the savannahs of Africa, millions of stomachs are busy converting plant tissue into animal flesh. The continent’s leaves and grasses are under constant assault from impala, wildebeest, buffalo, zebra, gazelles, and giraffes. Even acacia trees get bulldozed by elephants. There can be up to 25 species of these large plant-eaters in a given place, and many of them gather in gargantuan herds. How do they co-exist?
“It’s not obvious why competition for food doesn’t whittle the number of species down to just a few dominant competitors,” says Tyler Kartzinel from Princeton University. The prevailing idea says that different species have different food preferences. Grazers like zebra and wildebeest eat grass and little else. Browsers like dik-diks and giraffes nibble on leaves and shrubs—collectively called “browse”. Some animals, like elephants and impala, go for both.
Within each category, animals partition themselves in space. Zebras eat the tallest grasses; wildebeest munch the shorter ones. Dik-diks browse on the lowest leaves; impala take the mid-level; and giraffes pluck the loftiest foliage. But despite these nuances, “there’s still been this coarse distinction between grass and other plants,” says Kartzinel, “as if you partition those two resources finely enough, and suddenly there’s enough space in the savannah for dozens of herbivores.”
This picture is too simple. By using DNA to actually identify the plants that these animals eat—something no one had done before—Kartzinel has shown that their preferences go much deeper than just grass versus browse.